1. Field of the Invention
The present invention relates to an optical disc and an optical disc stamper for manufacturing such an optical disc.
2. Description of the Related Art
As an example of the related art, explanations are hereinafter given about how address information is recorded in prepits on a DVD-RAM, a rewritable optical disc with 2.6 GB recording capacity on one side thereof, by referring to FIGS. 5, 6, and 7.
The recording side of a DVD-RAM (2.6 GB) is divided into a plurality of cocentric areas which are called bands 501. Each band has tracks formed therein spirally, and a plurality of land tracks and groove tracks are disposed alternately per round and in succession. Each track is composed of a plurality of sectors which are data recording/playback units and each sector starts at the same position circumferentially as that of adjacent track sectors within the same band. At the top of each sector, a header which indicates synchronizing signals, address information and the like in the form of arranged prepits, and header areas 500 in a long strip shape in a radial direction are observed in appearance. An enlarged view of the header areas 500 is shown in FIG. 5. Recording areas of groove tracks 600 or prepits 603, when viewed through a base of the disc, are usually convex. Recording areas of the land tracks 601 are formed adjacent to the recording areas of the groove tracks 600.
When data is recorded/played back in the DVD-RAM drive, tracking servo is applied in a manner such that an optical spot follows a groove track center line 604 or a land track center line 605, and then a recording mark is written in the recording areas 600 and 601, and the data is read. A sector header is composed of arrays of the prepits 603, which are arranged on the border lines of the land tracks and the groove tracks. The header portion is divided into an ID 1-2 area and an ID 3-4 area, and each of them is arranged on the border lines of both sides.
In detecting these prepits using the DVD-RAM drive, a beam is converged at an objective lens 701, is transmitted through a base 701a1, and is then made to focus on a recording layer 703a1, and is further caused to track on the groove track center line 604 or the land track center line 605. The quantity of light of a reflection beam 700 is detected by using two-piece sensors 704 and 705 which are divided parallel to the track center line 605 and a header playback signal 707a2 is obtained by finding a difference between the outputs from the two sensors. When the light spot is located at the edge of a prepit 706, the luminous intensity distribution in the x axis direction of the reflection beam 700 will be as shown in distribution 707a1, and the output of the two-piece sensor 704 increases and becomes the state of point 708a3 of a header playback signal 710a3. On the other hand, when the light spot is not located at the edge of the prepit, the luminous intensity distribution in the x axis direction of the reflection beam 700 will be as shown in distribution 707a2 and, therefore, the outputs of the two-piece sensors 704 and 705 become the same and become the state of point 709a3 of the header playback signal 707a3.
With this conventional prepit arrangement in the DVD-RAM header area and by this detecting method, when the disc has a tilt in its radial direction or when a track offset is caused for some reason, the reading rate of the header playback signals suddenly decreases, and a tilt margin or an off-track margin becomes narrow. The reason for this is explained by using FIG. 7.
When there is no disc tilt as shown with regard to the header playback signal 710a3, an output signal is at 0 level in the state of 709a3 if the light spot is not located at a prepit, and high signal amplitude can be obtained.
When there is a disc tilt, however, the luminous intensity distribution of a reflection beam when the light spot is located at the edge of a prepit 706b1 is as shown in distribution 707b1, the peak of luminous intensity further shifting toward a negative direction compared to the distribution 707a1 and, therefore, a differential signal of the two-piece sensors 704 and 705 becomes the state of point 708b3 of the header playback signal 710b3, and remains at a lower level than that of point 708a3. When the light spot is not located at the edge of a prepit, the peak of the luminous intensity in the luminous intensity distribution of the reflection beam also shifts to a negative direction as shown in distribution 707b2, a differential signal of the two-piece sensors 704 and 705 becomes the state of point 709b3 of the header playback signal 710b3 and does not become a 0 level. Consequently, the playback signal amplitude decreases and the SN ratio (signal-to-noise ratio) lowers, thereby lowering the reading rate of the header signal and narrowing the margin for the disc tilt.
Likewise, when a track offset occurs, the signal amplitude decreases, thereby lowering the reading rate of the header signal and narrowing the margin for the track offset.
Therefore, it is an object of the present invention to enhance the reading rate of the header signal, and to provide an optical disc which can have a sufficient margin for the disc tilt or track offset.